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A brief browse inside a Model M7

  • Published 13 May 2020
  • Updated 07 August 2021
  • Est. Reading Time: 4.01 minutes

Here's a quick overview of what's inside an IBM Model M7. The Model M7 is a 49-key point of sale (POS) computer keypad originally produced by IBM between around 1994 until 2012, after which Toshiba continued using the design after buying out IBM's former Retail Store Solutions division. I've acquired three of these M7s over the last short while, hoping to produce PC-compatible macro pads from them to use at my home and my university office (once COVID-19 has buggered off). Being shown off today is the only one out of the three that is broken, serving as a spare parts bin and nothing more.

Just as an FYI: when the phrase "traditional Model M" is used, I am referring to a typical IBM Enhanced Keyboard such as part number 1391401.

Photo 41J7248_box_open.JPG

Starting with obviously the most interesting part, the keyboard is housed within two layers of box; an outer and very corporate (plain) looking box with a large IBM logo and an angular box around the M7 itself.

Photo 41J7248_box_close.JPG

M7s have two types of keycaps; 1-unit stems with either a 1-unit or 2-unit keycaps on top, or 1.25-unit stems with transparent keycaps on top. Not using a bar or rod stabiliser approach that traditional Model Ms used, the numeric zero key simply uses a second redundant stem for stabilisation.

Like traditional Model Ms, the keycaps are of a two-piece design
Like traditional Model Ms, the keycaps are of a two-piece design

Not being a consumer product, branding is tame on the M7s, with only a single IBM oval on the back wall.

Photo 41J7248_logo.JPG

M7s use an 8-pin variant of the shielded data link (SDL) connector that traditional older-generation Model Ms famously use in a 6-pin form. Compared to a standard SDL socket, this socket is overall wider and it is a very tight fit if you try and ram a standard SDL cable in there. It can get stuck, so don't try that at home. You have officially been warned! Besides, even if it did fit, it wouldn't do much as the M7 speaks RS485 serial, something no average home PC supports natively. Correction: it turns out this keypad can output AT scancodes, but I believe the MSR and lights functionality are still RS485 controlled via unused pins on the PS/2 connector.

Don't try inserting a normal IBM SDL cable into this
Don't try inserting a normal IBM SDL cable into this
Photo 41J7248_mount.JPG

One aspect M7s thankfully don't share with their more infamous brethren is that they are held together with just three Phillips screws instead of four 5.5mm nuts.

Photo 41J7248_screws.JPG
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The keyboard assembly is sandwiched just like a traditional M, with domes on top, followed by two layers of membrane, and backed by a thin plate of metal. The black coloured assembly on top of the plastic ridge is the magnetic stripe reader (MSR).

Guess the assembly is screw-modded for you, eh
Guess the assembly is screw-modded for you, eh
Photo 41J7248_mainboard_closeup.JPG

Perhaps the most interesting revelation for me is that the way the membrane connects to the mainboard is the same as how it is done on a traditional Model M. The type of connector is a Triomate, which is commercially available.

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have one such example of a commercial Triomate socket, and indeed, it can be used to mount the membrane to a different board. This fact is more commonly important in buckling springs circles for converting Wheelwriter Model M assemblies to USB, and converting these is exactly what I got in mind!

Specifically, the Triomate I'm holding is 6-520315-6
Specifically, the Triomate I'm holding is 6-520315-6

So, what do I think? Whilst this is not my first time opening an M7, it is definitely a different experience compared to most keyboard disassemblies. Given there is both a keypad and an MSR in there, there is obviously more going on inside this thing compared to most other keyboards. The inside is both interesting to look at and takes more time to understand what is going on. Anyway, these will make good project boards in the future!

Corrections & changes

  • 2021-08-07: I have since learned more about this keypad's SDL connection and its protocols, so I've inserted some details about my findings in the appropriate places in this article. Also, some minor grammar improvements were made.